The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
Heat sensitivity, instead of cold, is a symptom that is very specific to a tooth requiring root canal treatment. The tooth may start to hurt spontaneously, in the middle of the night, or sometimes when the patient isn't even using the affected tooth to eat or drink.